One use of magnetic bearings is in suspending the piston of Stirling cycle cryogenic refigerators in order to obtain long life. A major obstacle in the development of cryogenic cooler magnetic suspension systems has been the drift of the position sensor. It is a requirement of cryogenic magnetic suspension systems that their position be accurately measured. The presently used transducers for measuring shaft position are either eddy current sensors or capacitance sensors. Eddy current sensors drift as operating temperatures change. Capacitance sensors also do not possess long term stability. One reason is that surface contamination of the probe plates affect the dielectric constant between them. To achieve a high output sensitivity, capacitance type sensors must also be placed very close to the target. Moreover, the electronics employed with eddy current and capacitance position sensors also may tend to drift. All of these disadvantages are overcome with the instant invention. Even though reflective type optical sensors exist, none possess long term stability or high sensitivity.
Accordingly, it is an object of this invention to provide an optical position sensor for obtaining position information of a member.
It is a further object of this invention to provide an optical position sensor to obtain position information for active linear magnetic bearings.
It is yet a further object of this invention to provide an optical position sensor which possesses long term stability.
It is still another object of the invention to provide long term position stability by compensating for changes in the light source output, changes in detector sensitivity or drift, and changes in the transmission properties of the optical system.